Natural resources such as gas, oil, and water residing in a geological formation may be recovered by drilling a wellbore into the formation. A string of pipe (e.g., casing) is run into the wellbore in order to provide structural support for the wellbore sides. It is desirable to monitor the condition of the casing in the downhole environment. For example, it may be desirable to detect and locate leaks in the casing, tubing, or formation.